The present invention relates to gyroscopes, and more particularly to an angular velocity sensor using piezoelectric oscillating elements, and a sensor apparatus incorporating such sensor.
Conventionally, inertial navigation systems incorporating gyroscopes have been used to determine the bearing of a moving object such as an airplane or a ship. The gyroscopes include a sensor composed mainly of a spinning gyro of the mechanical type or a laser gyro using a laser apparatus. The mechanical or optical gyro is solely satisfactory in insuring a stable direction detection, however, it requires a relatively large and expensive structure and cannot, therefore, be applied to consumer equipment which should be small in size and of relatively low cost.
Another known angular velocity sensor is of the vibratory or oscillating type, which includes a sensor element co-oscillatable with an oscillating object or structure to which the sensor element is secured, for detecting the so-called Coriolis force. Most vibrational angular velocity sensors have either a piezoelectric mechanism or an electromagnetic mechanism. These sensors are characterized by the motion of a mass constituting a gyro which is a vibratory or oscillating motion rather than a rotary motion of a constant angular velocity. When an oscillating mass is subjected to an angular velocity .OMEGA., there is produced a force, known as the Coriolis force, in the form of a vibratory torque which is equivalent to the number of vibrations of the oscillating mass. One problem associated with such vibrational sensor is a high sensitivity to disturbing inertial forces.
When the sensor is subjected to a disturbing inertial force, a sensing signal produced by an oscillating sensor element involves a component dependent upon the Coriolis force and a component dependent upon the disturbing inertial force, which should be separated from one another. Since the Coriolis force is proportional to the vector product of an input inertial angular velocity and a driving velocity, this Coriolis force acts in a direction perpendicular to the direction of the instantaneous driving velocity and the direction of the input inertial angular velocity. If two such oscillating sensor elements are paired to oscillate in directions opposite to one another about a nodal axis of symmetry, disturbing inertial forces acting in a same direction can be canceled out by adding to a difference between two sensing electric signals, obtained by mechanical-to-electric conversion from forces acting on the respective oscillating sensor elements, a component corresponding to the Coriolis force acting in the opposite direction. Vibrational sensing of a torque caused by the Coriolis force provides a measurement of an angular velocity. This is the principle of operation of the vibratory angular velocity sensors stated above. Most vibratory angular velocity sensors devised heretofore utilize piezoelectric members as described for example in Journal of the Japan Society for Aerological and Space Sciences, Vol. 23, No. 257, pp. 339-350.
Japanese Patent Laid-open Publication No. 60-216210 in the name of the present assignee discloses an angular velocity sensor operative based on the principle described above. Generally speaking, a gyroscope relying on sensing of the Coriolis force in achieving a measurement of an angular velocity is a sensor which is ideally realized only when the assembling accuracy is theoretically within a zero tolerance zone. To this end, a vibratory angular velocity sensor using a pair of piezoelectric oscillating elements requires an extremely high precision assembly of the respective piezoelectric oscillating elements and various structural component parts to be assembled. It is however nearly impossible to assemble various component parts in an errorfree condition in terms of the parallelism, the perpendicularity and the degree of symmetry. As a result, a leak component signal is generated due to assembling tolerances. An inertial force (driving inertial force) acting as a reaction to an acceleration caused by driving or oscillation produces an unnecessary leak component signal in a direction perpendicular to a sense plane of the piezoelectric sensor elements.
Furthermore, in the vibratory angular velocity sensors, a sensing signal corresponding to the disturbing inertial force includes a leak component signal due to generation of an acceleration component caused by oscillation in a direction perpendicular to the sense axis. When the vibratory angular velocity sensor is used in a navigation system, the leak component signal appears as measurement errors which will significantly deteriorate the performance of the navigation system.
The leak component signal also varies with environmental conditions such as the temperature, time, etc. Accordingly, if a leak component signal resulting from assembling tolerances varies with a change in environmental conditions, the resulting leak component signal is detected and outputted from the sensor. The detected leak component signal causes a drift in the offset voltage. Furthermore, as widely known, the piezoelectric oscillating elements are pyroelectric and hence they produce an extremely large voltage when a temperature change takes place.